The present invention relates to a logic operation circuit having a sum output terminal and a carry output terminal.
Conventionally, as shown in FIG. 1, a full adder circuit which generates sum signals S and S and carry output signals CO and CO in response to logic input signals A and B and carry input signals C and C is known. The full adder circuit comprises transistors TR1 and TR2 having bases respectively connected to input terminals IT1 and IT2 for receiving the input signals A and B and collectors connected to a power source terminal VCC; transistors TR3 to TR8 having bases commonly connected to an input terminal IT3 for receiving the carry input signal C; and transistors TR9 to TR14 having bases commonly connected to an input terminal IT4 for receiving the carry input signal C. The collectors of the transistors TR3, TR6, TR10 and TR11 are commonly connected to the base of a transistor TR15 and are also commonly connected to the terminal VCC through a resistor R1. The collectors of the transistors TR4, TR5, TR9 and TR1 are commonly connected to the base of a transistor TR16 and are also commonly connected to the terminal VCC through a resistor R2. The collectors of the transistors TR7 and TR8 are commonly connected to the base of a transistor TR17 and are also connected to the terminal VCC through a resistor R3. The collectors of the transistors TR13 and TR14 are commonly connected to the base of a transistor TR18 and are also commonly connected to the terminal VCC through a resistor R4.
The collectors of the transistors TR15 to TR18 are commonly connected to the terminal VCC, and the emitters thereof are each commonly connected to a power source terminal VEE through resistors.
Furthermore, the full adder circuit comprises transistors TR19 to TR22 having bases commonly connected to the emitter of the transistor TR1; transistors TR23 to TR26 having bases commonly connected to a reference voltage terminal VR1; transistors TR27 and TR28 having bases commonly connected to the emitter of the transistor TR2 through a diode D; transistors TR29 and TR30 having bases commonly connected to a reference voltage terminal VR2; and transistors TR31 and TR32 having bases commonly connected to a reference voltage terminal VR3. The collectors of the transistors TR19, TR20, TR22, TR23, TR24 and TR25 are respectively connected to the emitters of the transistors TR3 and TR9, the emitters of the transistors TR5 and TR11, the emitters of the transistors TR8 and TR14, the emitters of the transistors TR4 and TR10, the emitters of the transistors TR6 and TR12, and the emitters of the transistors TR7 and TR13. The collectors of the transistors TR21 and TR26 are connected to the terminal VCC respectively through the resistors R3 and R4. The collectors of the transistors TR27 to TR32 are respectively connected to the emitters of the transistors TR19 and TR23, the emitters of the transistors TR21 and TR25, the emitters of the transistors TR20 and TR24, the emitters of the transistors TR22 and TR26, the emitters of the transistors TR27 and TR29, and the emitters of the transistors TR28 and TR30.
In the full adder circuit, the sum signal S and its inverted signal S thereof and the carry output signal CO and its inverted signal CO are generated from the emitters of the transistors TR16, TR15, TR18 and TR17 in accordance with the input signals A, B, C and C, respectively. Assume that the input signals A and B are at "1" level, and the carry input signals C and C are "0" and "1" levels. In this case, a current flows through the transistors TR9, TR19, TR27 and TR31, thereby rendering the transistors TR15 and TR16 conductive and nonconductive, respectively. Therefore, the "0" level sum signal S can be derived from the emitter of the transistor TR16. On the other hand, since the input signals A and B and the carry input signal C are "1" level, a current flows through the transistors TR21, TR28 and TR32, thus rendering the transistors TR18 and TR17 conductive and nonconductive, respectively. Therefore, the "1" level carry output signal CO can be derived from the emitter of the transistor TR18.
In the conventional full adder circuit, a circuit section for generating the sum signal and that for generating the carry signal are formed independently of each other. For this reason, the number of transistors required for constituting the full adder circuit is increased, and it is difficult to constitute an integrated circuit. Furthermore, in the conventional full adder circuit, since current paths of a number of transistors are connected in series between the terminals VCC and VEE, it is difficult to determine levels of input and reference voltages. In addition, this circuit cannot be driven at a low voltage.
A conventional full adder circuit comprising CMOS transistors having a first exclusive-OR circuit for receiving logic input signals A and B; a second exclusive-OR circuit for receiving an output signal from the first exclusive-OR circuit and a carry input signal C so as to generate a sum signal SO; and a selection circuit for selecting the input signal B or the carry signal C in accordance with the output signal from the first exclusive-OR circuit so as to generate the selected signal as a carry output signal CO has been proposed. The configuration of the full adder circuit is based on a relationship among the input signals A and B, the carry input signal C, an exclusive-OR value of the signals A and B, the sum signal SO and the carry output signal CO. Unlike a conventional full adder circuit comprising CMOS transistors, the number of elements is decreased, and the circuit is suited for integration. However, in this full adder circuit, since a number of MOS transistors are used, the operating speed is low